Prior art vinylidene fluoride base fluororubbers are used in a variety of applications mainly in the automobile and mechanical industries since they are elastomers having good heat resistance and mechanical strength.
These fluororubbers, however, are not satisfactory in chemical resistance. They are readily swollen in polar solvents such as ketone, lower alcohol, and organic acid solvents. They are deteriorated by amine-containing chemicals so that their rubber strength and elongation may substantially decline. With respect to low-temperature properties, the fluororubbers lose rubber elasticity at temperatures below −20° C. to such an extent that they cannot be used as sealants. This generally imposes a limit on the use of fluororubbers in cold areas.
In order to eliminate these drawbacks, curable fluorine-containing compositions comprising a perfluoro compound and a fluorinated organohydrogenpolysiloxane as main components were proposed. These compositions, however, are liquid because they are based on liquid perfluoro compounds having a low degree of polymerization. Then the compositions are suitable in the formed in-place gasket (FIPG) process and liquid injection molding system (LIMS), but less effective in compression molding commonly employed for rubber molding.
In particular, the curable fluorine-containing compositions often prohibit the use of conventional two-plate molds for rubber because of the difficulty of molding and the frequent occurrence of defects by air entrapment. Steady production is difficult unless a special LIMS mold is newly fabricated. The LIMS mold, however, has the problems that it is generally more expensive than the conventional two-plate molds for rubber, the mounting of the mold in a LIMS molding machine is cumbersome, and the molding machine requires time-consuming adjustment after mold mounting. The LIMS mold does not lend itself to the manufacture of a variety of parts in small quantities.
Under the circumstances, the assignee previously proposed in JP-A 2001-164115 (corresponding to U.S. Ser. No. 09/729,774 or US 2001-0008914A) a method for preparing a rubber composition of the type that can be roll milled and molded in a compression molding rubber mold, referred to as millable composition, from a fluorinated curable composition comprising a perfluoro compound and a fluorinated organohydropolysiloxane as main components.
A method of preparing this composition involves adding part of a crosslinker to a liquid polymer for reaction to take place to form a gel-like gum, then adding a filler to the gum so that the liquid polymer is converted to a millable polymer. The filler must be added to and compounded with the gel-like gum rather than usual millable gum. Compounding of the filler with the gel-like gum is time consuming. This is undesirable when the method is scaled up for a mass scale of production.